Semiconductor laser device and manufacturing method of the same

ABSTRACT

A semiconductor laser device includes an active layer formed on a substrate, and current blocking layers formed on the substrate so as to sandwich the active layer. Each current blocking layer has a low impurity concentration at a portion near the active layer and a high impurity concentration at a portion apart from the active layer. A manufacturing method of the semiconductor laser device is also disclosed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor laser device anda manufacturing method of the same and, more particularly, to ahigh-output semiconductor laser device and a manufacturing method of thesame.

[0003] 2. Description of the Prior Art

[0004] As shown in FIG. 1A, a conventional semiconductor laser is formedby selectively stacking an n-InP cladding layer 22, i-InGaAsP activelayer 23, and p-InP cladding layer 24 on an n-InP substrate 21,selectively growing p-InP current blocking layers 25 and n-InP currentblocking layers 26 constituting a pnpn thyristor structure on the twosides of the stacked structure, and forming a p-InP over cladding layer27 and p-InGaAs capping layer 28 on the n-InP current blocking layers26. In this semiconductor laser, the current blocking layers 25 formedon the two sides of the i-InGaAsP active layer 23 serving as a lightemission region effectively confine an injection current in thei-InGaAsP active layer 23. The p-InP current blocking layers 25 play animportant role for a high laser output. To enhance the current blockingeffect, the current blocking layer must be heavily doped with animpurity.

[0005] In this conventional semiconductor laser, the p-InP currentblocking layer 25 is heavily doped with an impurity to ensure a highbreakdown voltage of the current blocking layer 25. In this case,however, since the p-InP current blocking layer 25 near the i-InGaAsPactive layer 23 grows on a high-order plane such as the (311) plane or(211) plane which easily entraps an impurity element, the impurityconcentration near the i-InGaAsP active layer 23 increases. That is, inthe semiconductor laser, since the migration effect is prompted to growthe p-InP current blocking layer 25, as shown in FIG. 1B, the p-InPcurrent blocking layer 25 near the i-InGaAsP active layer 23 grows onthe (111)B plane at the start of growth, and sequentially grows onhigh-order planes such as the (311) plane and (211) plane owing to themigration effect.

[0006] In a general semiconductor laser, light emitted from thei-InGaAsP active layer 23 enters the p-InP current blocking layer 25 toa certain degree. Thus, if the impurity concentration of the p-InPcurrent blocking layer 25 near the i-InGaAsP active layer 23 is high,the free carrier absorption loss in the p-InP current blocking layer 25increases, failing to attain a high output. In addition, a high impurityconcentration near the i-InGaAsP active layer 23 decreases theresistance, and a leakage current from the p-InP cladding layer 24 onthe i-InGaAsp active layer 23 to the p-InP current blocking layer 25tends to increase. This leakage current serves as a gate current to thepnpn current blocking thyristor, and a larger leakage current decreasesthe breakdown voltage of the blocking layer. Further, the impurity ofthe p-InP current blocking layer 25 is diffused to the i-InGaAsP activelayer 23 to degrade characteristics and reliability due to impuritycontamination.

SUMMARY OF THE INVENTION

[0007] The present invention has been made in consideration of the abovesituation, and has as its object to provide a high-output semiconductorlaser device with high reliability which can reduce the free carrierabsorption loss in a current blocking layer and the leakage current, anda manufacturing method of the same.

[0008] To achieve the above object, according to the first aspect of thepresent invention, there is provided a semiconductor laser devicecomprising an active layer formed on a substrate, and current blockinglayers formed on the substrate so as to sandwich the active layer,wherein each current blocking layer has a low impurity concentration ata portion near the active layer and a high impurity concentration at aportion apart from the active layer.

[0009] In the first aspect, the portion of the current blocking layernear the active layer is a region 1 to 2 μm and preferably not more than1 μm apart from an end of the active layer.

[0010] In the first aspect, the current blocking layer has an impurityconcentration of 3 to 5×10¹⁷ cm⁻³ at the portion near the active layer,and an impurity concentration of 7 to 10×10¹⁷cm⁻³ at the portion apartfrom the active layer.

[0011] To achieve the above object, according to the second aspect ofthe present invention, there is provided a manufacturing method of asemiconductor laser device, comprising the steps of forming an activelayer on a substrate, and selectively forming current blocking layers onthe substrate by metal organic vapor phase epitaxy so as to sandwich theactive layer, wherein a growth condition along the metal organic vaporphase epitaxy of each current blocking layer is determined such that animpurity concentration is low at a portion near the active layer andhigh at a portion apart from the active layer in the step of forming thecurrent blocking layers.

[0012] In the second aspect, the first growth condition along the metalorganic vapor phase epitaxy of the current blocking layer is to set agrowth temperature in an initial stage of metal organic vapor phaseepitaxy for the current blocking layer to be lower than a subsequentgrowth temperature. The second growth condition is to set a growthpressure in the initial stage of metal organic vapor phase epitaxy forthe current blocking layer to be higher than a subsequent growthpressure. The third growth condition is to set a growth rate in theinitial stage of metal organic vapor phase epitaxy for the currentblocking layer to be higher than a subsequent growth rate. The fourthgrowth condition is to continuously supply a source gas of Group V inthe initial stage of metal organic vapor phase epitaxy for the currentblocking layer and to subsequently intermittently supply the source gasof Group V. The fifth growth condition is to increase a ratio of asource gas of Group III to a source gas of Group V in the initial stageof metal organic vapor phase epitaxy for the current blocking layer andto subsequently decrease the ratio.

[0013] As is apparent from these aspects, in the semiconductor laserdevice of the present invention, the impurity concentration of thecurrent blocking layer near the active layer can be decreased withoutdecreasing the breakdown voltage of the current blocking layer. Hence,the free carrier absorption loss in the current blocking layer can bereduced to obtain a high optical output and high reliability.

[0014] By adopting the manufacturing method of the semiconductor laserdevice according to the present invention, for example, the migrationeffect of a source species is suppressed at a portion near an i-InGaAsPactive layer in the initial growth stage during formation (growth) of ap-InP current blocking layer. The p-InP current blocking layer keepsgrowing on the (111)B plane. To the contrary, the migration effect ofthe source species is prompted by changing crystal growth conditions ata portion apart from the active layer in the latter growth stage, andthe current blocking layer grows on high-order planes such as the (311)plane and (211) plane. When the crystal growth plane is a high-orderplane such as the (311) plane or (211) plane, a dangling bond is morereadily formed during growth, and a larger amount of impurity isentrapped, compared to the (111)B plane having a very low growth rate.Therefore, the p-InP current blocking layer has a low impurityconcentration near the active layer and a high impurity concentration ata portion apart from the active layer.

[0015] As described above, according to the present invention, since theimpurity concentration of the current blocking layer near the activelayer is relatively low, the free carrier absorption loss duringpropagation of light emitted by the active layer can be reduced. Inaddition, since the impurity concentration of the current blocking layerapart from the active layer is relatively high, the breakdown voltage ofthe current blocking layer can be kept high. Since the impurityconcentration of the current blocking layer near the active layer isrelatively low, the resistance near the active layer is high, and theleakage current flowing from an over cladding layer immediately abovethe active layer to the current blocking layer via a cladding layer canbe reduced to increase the breakdown voltage of the current blockinglayer. This can realize a high efficiency and high breakdown voltage,and can provide a high-optical-output semiconductor laser device.Moreover, since the impurity concentration of the current blocking layerin contact with the active layer is relatively low, diffusion of theimpurity to the active layer can be suppressed. A semiconductor laserdevice with high reliability can be obtained.

[0016] The above and many other objects, features and advantages of thepresent invention will become manifest to those skilled in the art uponmaking reference to the following detailed description and accompanyingdrawings in which preferred embodiments incorporating the principle ofthe present invention are shown by way of illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1A is a sectional view showing an example of a conventionalsemiconductor laser device;

[0018]FIG. 1B is a sectional view showing the main part forschematically explaining a manufacturing method of a current blockinglayer in the conventional semiconductor laser device;

[0019]FIGS. 2A to 2D are sectional views, respectively, showing thesteps in a manufacturing method of a semiconductor laser deviceaccording to the present invention; and

[0020]FIGS. 3A and 3B are sectional views for schematically explaining amanufacturing method of a current blocking layer in the semiconductorlaser device according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0021] Several preferred embodiments of the present invention will bedescribed below with reference to the accompanying drawings.

[0022]FIGS. 2A to 2D are sectional views, respectively, showing thesteps in manufacturing a semiconductor laser device according to thepresent invention. As shown in FIG. 2A, a pair of SiO₂ stripe masks 18are formed on an n-InP substrate 11, and used as growth stopper films toselectively form an n-InP cladding layer 12, i-InGaAsP active layer 13,and p-InP cladding layer 14 by metal organic vapor phase epitaxy(MOVPE). As shown in FIG. 2B, the SiO₂ growth stopper films 18 areremoved, and an SiO₂ growth stopper film 19 is formed on only the p-InPcladding layer 14. P-InP current blocking layers 15 and n-InP currentblocking layers 16 are selectively grown by MOVPE. This growth step willbe described in detail.

[0023]FIGS. 3A and 3B are enlarged views, respectively, showing a region20 in FIG. 2B.

[0024] In the first embodiment, the growth temperature is set to 570° C.near the i-InGaAsP active layer 13, i.e., in the initial growth stageduring the growth of the p-InP current blocking layer 15. This reducesthe kinetic energy of a source species to suppress the migration effect.The p-InP current blocking layer 15 near the i-InGaAsP active layer 13keeps growing on the (111)B plane. When the p-InP current blocking layer15 grows to 1 to 2 μm and preferably 1 μm from the end of the i-InGaAsPactive layer 13, i.e., in the latter growth stage, the growthtemperature is increased to 650° C., thereby increasing the kineticenergy of the source species and prompting the migration effect. Thesource species having reached the p-InP current blocking layer formed onthe (111)B plane does not grow on the (111)B plane having a very lowgrowth rate, but migrates on the surface to sequentially grow onhigh-order planes such the (311) plane and (211) plane. When the crystalgrowth plane is a high-order plane such as the (311) plane or (211)plane, a dangling bond is readily formed during growth, and a largeramount of impurity (Zn in this embodiment) is entrapped. As a result,the impurity concentration of the p-InP current blocking layer 15 can beselectively changed to 3 to 5×10¹⁷ cm⁻³ near the i-InGaAsP active layer13 and 7 to 10×10¹⁷ cm⁻³ in a region 1 to 2 μm apart from the i-InGaAsPactive layer 13.

[0025] After the p-InP current blocking layers 15 have grown, n-InPcurrent blocking layers 16 are formed on them, as shown in FIG. 2C. TheSiO₂ growth stopper film 19 is removed, and a p-InP over cladding layer17 and p-InGaAs capping layer 18 are grown on the entire surface byMOVPE, as shown in FIG. 2D, thereby completing the semiconductor laserdevice of the present invention.

[0026] In the semiconductor laser device manufactured in this manner,since the impurity concentration of the p-InP current blocking layer 15near the i-InGaAsP active layer 13 is relatively low, the lightabsorption loss during propagation of light emitted by the i-InGaAsPactive layer 13 can be reduced. In addition, since the impurityconcentration of the p-InP current blocking layer 15 apart from thei-InGaAsP active layer 13 is high, the breakdown voltage of the currentblocking layer can be kept high. Since the impurity concentration nearthe i-InGaAsP active layer 13 is relatively low, as described above, theresistance near the i-InGaAsP active layer 13 is high, and the leakagecurrent flowing from the p-InP over cladding layer 17 to the p-InPcurrent blocking layer 15 via the p-InP cladding layer 14 can bereduced. This leakage current serves as a gate current to the pnpncurrent blocking thyristor, so that a smaller leakage current canincrease the breakdown voltage of the current blocking layer. Theseadvantages can realize a high efficiency, high breakdown voltage, andhigh optical output. Moreover, since the impurity concentration of thep-InP current blocking layer 15 in contact with the i-InGaAsP activelayer 13 is low, diffusion of the impurity to the i-InGaAsP active layer13 can be suppressed. The semiconductor laser device can, therefore,obtain high reliability. A 1.48-μm semiconductor laser device with aconventional structure attains only an optical output of 180 mW for anoscillation threshold of 30 mA and a slope efficiency of 0.380 W/A and500 mA. To the contrary, a 1.48-μm semiconductor laser device accordingto the first embodiment can attain an optical output of 200 mW for anoscillation threshold of 20 mA and a slope efficiency of 0.450 W/A and500 mA.

[0027] The second to fifth embodiments in which the p-InP currentblocking layer 15 is formed at a low impurity concentration near thei-InGaAsP active layer 13 and a high impurity concentration in a distantregion will be described. In the second embodiment, while a p-InPcurrent blocking layer 15 adjacent to an active layer is selectivelygrown by MOVPE, the growth pressure is relatively increased near ani-InGaAsP active layer 13 in the initial growth stage. For example, thegrowth pressure is increased to 150 Torr. This shortens the mean freepaths of atoms and molecules to suppress the migration effect. Underweak migration effect, the p-InP current blocking layer 15 grows on the(111)B plane. In the latter growth stage, i.e., in a region apart fromthe i-InGaAsP active layer 13, the growth pressure is decreased to,e.g., 50 Torr. This suppresses decomposition of a source gas of GroupIII to prompt the migration effect on a semiconductor crystal surface.Under strong migration effect, the element which has reached the (111)Bplane does not grow on the (111)B plane having a very low growth rate,but migrates on the semiconductor surface to sequentially grow onhigh-order planes such as the (311) plane and (211) plane. Thismanufacturing method can form the p-InP current blocking layer 15 whichhas a low concentration near the active layer and a high concentrationin a region distant from the active layer on the basis of the sameprinciple as the first embodiment.

[0028] In the third embodiment, while a p-InP current blocking layer 15adjacent to an i-InGaAsP active layer 13 is selectively grown by MOVPE,the growth rate is relatively increased near the i-InGaAsP active layer13 in the initial growth stage. For example, the growth rate isincreased to 1.5 μm/h. A high growth rate is obtained by increasing theflow rate of a source gas of Group III. As a result, the mean free pathof atoms of Group III is shortened to suppress the migration effect.Under weak migration effect, the p-InP current blocking layer grows onthe (111)B plane. In the latter growth stage, i.e., in a region apartfrom the active layer 13, the growth rate is relatively decreased to,e.g., 0.75 μm/h. This prompts the migration effect on a semiconductorcrystal surface. Under strong migration effect, the element which hasreached the (111)B plane does not grow on the (111)B plane having a verylow growth rate, but migrates on the semiconductor surface tosequentially grow on high-order planes such as the (311) plane and (211)plane. This manufacturing method can form the p-InP current blockinglayer 15 which has a low concentration near the active layer 13 and ahigh concentration in a region distant from the active layer 13 on thebasis of the same principle as the first embodiment.

[0029] In the fourth embodiment, while a p-InP current blocking layer 15adjacent to an i-InGaAsP active layer 13 is selectively grown by MOVPE,a source gas of Group V is continuously supplied near the i-InGaAsPactive layer 13 in the initial growth stage. This prompts decompositionof the source gas of Group III to suppress the migration effect. Underweak migration effect, the p-InP current blocking layer 15 grows on the(111)B plane. In the latter growth stage, i.e., in a region apart fromthe active layer 13, the source gas of Group V is intermittentlysupplied. For example, crystal growth is done in the cycle of a 1-secsupply time and 2-sec idle time. During the idle time, the mean freepath of the element of Group III elongates to prompt the migrationeffect on a semiconductor crystal surface. Under strong migrationeffect, the element which has reached the (111)B plane does not grow onthe (111)B plane having a very low growth rate, but migrates on thesemiconductor surface to sequentially grow on high-order planes such asthe (311) plane and (211) plane. This manufacturing method can form thep-InP current blocking layer 15 which has a low concentration near theactive layer 13 and a high concentration in a region distant from theactive layer 13 on the basis of the same principle as the firstembodiment.

[0030] In the fifth embodiment, while a p-InP current blocking layer 15adjacent to an i-InGaAsP active layer 13 is selectively grown by MOVPE,the ratio (V/III) of a source gas of Group V to a source gas of GroupIII is increased near the i-InGaAsP active layer 13 in the initialgrowth stage. For example, the ratio V/III is increased to 500. Thisprompts decomposition of the source gas of Group V, increases thepartial pressure of the element of Group V, shortens the mean free pathof the element of Group III, and suppresses the migration effect. Underweak migration effect, the p-InP current blocking layer 15 grows on the(111)B plane. In the latter growth stage, i.e., in a region apart fromthe active layer 13, the ratio V/III is decreased to, e.g., 100. Thissuppresses decomposition of the source gas of Group III to prompt themigration effect on a semiconductor crystal surface. Under strongmigration effect, the element which has reached the (111)B plane doesnot grow on the (111)B plane having a very low growth rate, but migrateson the semiconductor surface to sequentially grow on high-order planessuch as the (311) plane and (211) plane. This manufacturing method canform the p-InP current blocking layer 15 which has a low concentrationnear the active layer 13 and a high concentration in a region distantfrom the active layer 13 on the basis of the same principle as the firstembodiment.

What is claimed is:
 1. A semiconductor laser device comprising an activelayer formed on a substrate, and current blocking layers formed on thesubstrate so as to sandwich said active layer, wherein each currentblocking layer has a low impurity concentration at a portion near saidactive layer and a high impurity concentration at a portion apart fromsaid active layer.
 2. A device according to claim 1, wherein the portionof said current blocking layer near said active layer is a region 1 to 2μm and preferably not more than 1 μm apart from an end of said activelayer.
 3. A device according to claim 1, wherein said current blockinglayer has an impurity concentration of 3 to 5×10¹⁷ cm⁻³ at the portionnear said active layer, and an impurity concentration of 7 to 10×10¹⁷cm⁻³ at the portion apart from said active layer.
 4. A manufacturingmethod of a semiconductor laser device, comprising the steps of formingan active layer on a substrate, and selectively forming current blockinglayers on the substrate by metal organic vapor phase epitaxy so as tosandwich the active layer, wherein a growth condition is determined inaccordance with a metal organic vapor phase epitaxy stage of eachcurrent blocking layer such that the current blocking layer near theactive layer has a low impurity concentration in an initial stage ofmetal organic vapor phase epitaxy for the current blocking layer, andthe current blocking layer apart from the active layer has a highimpurity concentration in a subsequent metal organic vapor phase epitaxystage.
 5. A method according to claim 4, wherein the growth condition isto set a growth temperature in the initial stage of metal organic vaporphase epitaxy for the current blocking layer to be lower than asubsequent growth temperature.
 6. A method according to claim 4, whereinthe growth condition is to set a growth pressure in the initial stage ofmetal organic vapor phase epitaxy for the current blocking layer to behigher than a subsequent growth pressure.
 7. A method according to claim4, wherein the growth condition is to set a growth rate in the initialstage of metal organic vapor phase epitaxy for the current blockinglayer to be higher than a subsequent growth rate.
 8. A method accordingto claim 4, wherein the growth condition is to continuously supply asource gas of Group V in the initial stage of metal organic vapor phaseepitaxy for the current blocking layer and to subsequentlyintermittently supply the source gas of Group V.
 9. A method accordingto claim 4, wherein the growth condition is to increase a ratio of asource gas of Group III to a source gas of Group V in the initial stageof metal organic vapor phase epitaxy for the current blocking layer andto subsequently decrease the ratio.
 10. A method according to claim 5,wherein the growth temperature in the initial stage of metal organicvapor phase epitaxy for the current blocking layer is 570° C., and thesubsequent growth temperature is 650° C.
 11. A method according to claim6, wherein the growth pressure in the initial stage of metal organicvapor phase epitaxy for the current blocking layer is 150 Torr, and thesubsequent growth pressure is 50 Torr.
 12. A method according to claim7, wherein the growth rate in the initial stage of metal organic vaporphase epitaxy for the current blocking layer is 1.5 μm/h, and thesubsequent growth rate is 0.75 μm/h.
 13. A method according to claim 8,wherein the source gas of Group V is intermittently supplied in a cycleof a 1-sec supply time and 2-sec idle (non-supply) time.
 14. A methodaccording to claim 9, wherein the ratio of the source gas of Group IIIto the source gas of Group V in the initial stage of metal organic vaporphase epitaxy for the current blocking layer is 500, and the subsequentratio is
 100. 15. A method according to claim 4, wherein the initialstage of metal organic vapor phase epitaxy for the current blockinglayer is a time required for growing the current blocking layer from anend of the active layer to 1 to 2 μm and preferably not more than 1 μm.